The conventional mobile communication networks are mainly Circuit Switched networks, including GSM and CDMA. Operators have established well-developed and abundant-service platforms based on the CS network, where a Mobile Switch Center (MSC) is in charge of call routing and service logic execution (for example, executing a forwarding service).
With the popularity and development of the network IP technology, IP multimedia service attracts more and more attentions, and the IP Multimedia Sub-system (IMS) for providing IP multimedia services is also promoted and developed. The IMS supports User Equipment (UE) to access the IMS to perform IMS services through various Packet Switched (PS) networks. Current PS networks are mainly IP capability access networks (IP-CAN) (such as the GPRS). The IMS is a service platform based on the IP-CAN, and has similar functions with the MSC of the CS network.
In view of the development trend, the IMS-based network will finally complete comprehensive deployment to replace the CS network. However, the IMS-based network is complex, and the complete deployment cannot be achieved in a short period of time, and moreover, it is unpractical to require all the CS network users to replace a new IMS terminal in a short period of time. Therefore, the IMS-based network will co-exist with the conventional CS network for a long time in the future. Under such condition, the simultaneous operation of the CS system and the IMS system will increase operation costs. Furthermore, if a new service is introduced, the CS system and the IMS system both need to be re-constructed, thus increasing construction costs. Therefore, a good method for solving the above problem is to transfer the function of the CS network system to the IMS network system to achieve uniformity between the systems, so as to reduce operation costs and construction costs for new services. Such a uniform system is referred to as IMS Centralized Service (ICS).
FIG. 1 shows the structure of an ICS system. As shown in FIG. 1, the network side includes a Server Centralization and Continuity Application Server (SCC AS), a Call Session Control Function (CSCF), a CS Access interface, an MSC Server, and a CS Media Gateway (CS-MGW).
The SCC AS is configured to provide an application server supported by the ICS service and perform the core ICS service logic control. The CSCF provides the core IMS session control function, and includes a Proxy-CSCF (P-CSCF), a Service-CSCF (S-CSCF), and an Interrogating-CSCF (I-CSCF). In the ICS system, the UE is required to establish an IMS call with voice media over a CS bearer, and the CS bearer is achieved by establishing a CS call between the UE and the SCC AS, in addition, the service processing logic in the MSC is weakened or removed. The SCC AS completes the adaptation from the CS domain signalling to the IMS domain Session Initial Protocol (SIP) signalling, and as a user agent, establishes and controls the IMS session in the IMS domain for the user. Between the UE and the SCC AS, a session control message is transferred through a signalling channel other than the CS call, and in different access network environments, different message transmission protocols may be used. When only the CS domain connection exists, the UE and the SCC AS perform information exchange through a Service Control Signalling Path (that is, I1 reference point) of the CS network. When the UE is in an IP-CAN connection without the Voice over Internet Protocol (VoIP) capability, the UE and the SCC AS perform information exchange through a Service Control Signalling Path (that is, Gm reference point) of the PS network. Compared with the I1 reference point, the Gm reference point has the advantages that large amount of information and rich content can be transmitted. In communication standards, it is recommended that when the Gm reference point and the I1 reference point are both available, the Gm reference point is preferred as a channel for session control message transmission.
In the implementation of the present disclosure, the inventor finds that the process of information exchange through the Gm reference point in the prior art at least has the following problems:
When the PS network where the Gm reference point is located is interrupted and thus is unavailable, the UE abandons the Gm reference point and uses the I1 reference point instead; when the amount of transmitted information is large and there is much content to be transmitted, such operation mode results in low transmission efficiency, and even some information cannot be transmitted.